Carbon allotropes can be classified according to the carbon atom hybridization. In principle, there are different ways, based on various parameters, such as range dimensionality, type of chemical bonds, etc. which can be used to classify carbon nanostructures. Classifications vary function of the field of nanostructure applications. In a point of view, one can classify the carbon allotropes by the type of carbon atom hybridation. This chapter is a brief review introduction to some major allotropes: graphene/graphite, carbon nanotubes, diamond and amorphous carbon. In addition, Chemical Vapor Deposition (CVD) techniques, frequently used for synthesizing these structures are discussed. The influence of some important experimental parameters on the growth of high quality diamond and diamond-like carbon DLC are also investigated.
TopIntroduction
Carbon is one of the most important natural elements in the Periodic Table which never stops to surprise the world with its various allotropes (see Figure 1). There exist millions of different compounds with fantastic chemical and physical properties of its different forms (Balaban, 2015). This unique element among all the other ones, has four valence electrons, two in the 2s sub-shell and two in the 2p sub-shells, and four vacancies in its outer shells, with a ground state electron distribution of 1s2 2s2 2p2. Such atomic arrangement enables it the building of several allotropic forms in solid state, such as graphene (Monajjemi, 2014), graphite (Matsumoto et al., 2009), diamond (Khalaj et al., 2010; Atefi et al., 2010; Shahsavari et al., 2011; Khalaj et al., 2012a), diamond-like carbon (Khalaj et al.,2012b; Vaghri et al., 2011), fullerene (Komatsu et al., 2005), carbon nanotubes (Shams et al., 2012) and carbon nanowalls (Vizireanu et al., 2010), etc. The different types of carbon allotropes have a great influence on material’s properties, some of them existing in the nature (either on earth or extraterrestrial) but also in the synthesized molecules. More than 100 years ago, when Edison used a hot incandescent carbonaceous filament in his experiment, many forms of carbon have been prepared or discovered in laboratory.
Figure 1.
Some of the carbon allotropes
In this chapter, there is a short description on some of the main allotropes of carbon. In addition, a particular view to diamond, in both theoretical and experimental outlook, is provided. Overall, a great variety of materials are formed by carbon, ranging from crystalline to amorphous structures (Khalaj et al., 2010). The two famous primary crystalline forms of the carbon are the diamond (100% of sp3 bonding) and graphite (with 100% of sp2 bonding), while the amorphous carbon contains a mixture of sp3 and sp2 bonding. Graphite is the most stable phase of carbon (in a large range of conditions); it is made of hundreds of thousands of layers of the planar polyhex network. By scrolling a graphene sheet, a carbon nanotube can be formed (even the process is not so simple).